It's a continuum. Respecting software freedom is the most ethical action, so using proprietary terms is less ethical. The software may, depending on your view, not be net unethical at that point. It's further unethical if they have problematic terms in the EULA besides being proprietary in the first place. Competing with free software and thus taking attention and therefore resources away from the free software is negative ethically, in the same sense that advertising bottled water is unethical even if it didn't have the environmental problems that it has. It's especially unethical if software spies on you and advertises at you or even worse things.

Implying that non-free software is unethical isn't rude. Especially since I didn't insist that this particular software is deeply unethical. I just suggested that support for ethical software like GIMP is the right thing to do and so supporting less ethical software is something I'm suggesting avoiding.

Now, having learned from other comments about this particular case, I'll agree that as proprietary software goes, Paint.net sounds unusually good in the ethics realm, just not quite on par with GIMP, but I'd say net ethical after all.